A navigation apparatus provides a user with guidance for traveling on correct routes, so that the user is free from having to constantly look at maps and watch for road signs. However, Global Positioning System (GPS) signals which the navigation system largely relies on may easily be shielded by buildings and landforms resulting in poor reception quality in certain areas. Especially when the number of visible satellites for the navigation apparatus is less than four or when the navigation system is located at a weak-signal region, the navigation apparatus becomes incapable of providing required positioning data. Under such circumstances, even if positioning signals are received, it is possible that positioning values calculated from the received GPS signals are quite different from actual values such that a current location may be misjudged.
To improve positioning accuracy, current research has mainly focused on differential GPS (DPGS), dead reckoning (DR), inertial navigation systems (INS) and map matching (MMT) to provide a better solution.
How a navigation apparatus of the prior art utilizes the MMT function for performing positioning calculations shall be discussed below. Referring to FIG. 1, a navigation apparatus 1 comprises a GPS receiver 10, a computing unit 12, a first storage medium 14, a second storage medium 16 and a display device 18. The GPS receiver 10 receives a GPS signal P1. The first storage medium 14 stores a previous position-velocity-time (PVT) data P2(t−1). Note that PVT data contain information of position, velocity and time offset. The second storage medium 16 stores map data P4. The computing unit 12 computes a current PVT data P2(t) according to the GPS signal P1 and the previous PVT data P2(t−1). According to the map data P4 and the computed current PVT data P2, the computing unit 12 then computes a current MMT result P3(t). At this point, the display device 18 displays the map data P4 and the current MMT result P3(t) acquired by the computing unit 12.
In an MMT algorithm, a matching operation is performed between the map data P4 and the current positioning parameter P2(t), so as to calibrate the current PVT data P2(t) onto a current route and increase positioning accuracy. However, since the positioning computations of the navigation apparatus 1 are solely based on the previous PVT data P2(t−1) and the GPS signal P1, and due to significant errors in the imported values, the navigation apparatus 1 can hardly calibrate the current PVT data P2(t) onto the correct route even if the current PVT data P2(t) is calibrated using MMT. In addition, since the current PVT data P2(t) acquired from the computation at a current time point includes a certain degree of error, which is accounted when the current PVT data P2(t) is imported as a previous PVT data P2(t−1), a computation at a next time point to lead to accumulative errors. More specifically, in the prior art, in addition to the computed values imported by the navigation apparatus 1 being not considered accurate enough, the errors are also accumulated along with time such that the current MMT result P3(t) acquired is not likely to reflect a correct route.
Therefore, the present invention provides a navigation apparatus and a positioning method thereof, which are capable of increasing positioning accuracy for overcoming the aforesaid issues.